Coating type magnetic recording media, that is, magnetic recording media comprising a nonmagnetic support and a magnetic layer formed on the support and comprising magnetic powder and a binder, are required to have a further increased recording density with the shift of a writing-reading system from an analog system to a digital one. In particular, such requirement has been increased year by year in the video tapes and the backup tapes for computers which are used for high density recording.
To cope with short wavelength recording which is inevitable to increase a recording density, it is effective to decrease the thickness of a magnetic layer to 300 nm or less, in particular, to 100 nm or less so as to decrease a thickness loss during recording. In general, a magneto resistance head (MR head) is used as a reproducing head which is used in combination with such high density recording media.
The particle size of magnetic powder has been decreased year by year to reduce a noise. Nowadays, acicular metal magnetic powder having a particle size of about 100 nm is practically used. Furthermore, to prevent the decrease of output caused by demagnetization during short wavelength recording, the coercive force of the magnetic powder has been increased, and a coercive force of about 238.9 A/m (about 3,000 Oe) is realized with an iron-cobalt alloy (see JP-A-3-49026, JP-A-10-83906 and JP-A-10-34085).
However, a coercive force depends on the shape of acicular magnetic particles in a magnetic recording medium comprising acicular magnetic particles. Thus, it is difficult to further decrease the particle size of such acicular magnetic particles. That is, if the particle size is further decreased, a specific surface area greatly increases and saturation magnetization greatly decreases. Consequently, the high saturation magnetization, which is the most significant characteristic of metal or metal alloy magnetic powder, is deteriorated, so that the use of the metal or metal alloy becomes meaningless.
In view of the above circumstance, a magnetic recording medium using, as a magnetic powder which is totally different from the acicular magnetic powder, a rare earth element-transition metal particulate magnetic powder such as a spherical or ellipsoidal rare earth element-iron-boron magnetic powder is proposed (see JP-A-2001-181754). This medium can greatly decrease the particle size of the magnetic powder and achieve a high saturation magnetization and a high coercive force. Therefore, this medium significantly contributes to the increase of a recording density.
Also, a magnetic recording medium using, as an iron magnetic powder having a non-acicular particle shape, an iron nitride magnetic powder which comprises random shape particles and a Fe16N2 phase as a main phase, and has a BET specific surface area of about 10 m2/g is proposed (see JP-A-2000-277311).
However, the rare earth element-iron-boron magnetic powder of JP-A-2001-181754 is a composite material which comes into existence based on the balance of a high magnetic anisotropy due to the rare earth element compound and high saturation magnetization due to the iron material which forms the cores of the magnetic particles. If such a magnetic powder is further improved, for example, the coercive force of such a magnetic powder is intended to be further increased, it is very difficult to improve the magnetic properties while maintaining the optimum dispersibility and chemical stability of the magnetic recording medium. JP-A-2000-277311 discloses the iron nitride magnetic powder having a BET specific surface area of 10 to 22 m2/g in the Example. However, such magnetic powder has a very large particle size and is not suitable for high density magnetic recording with achieving the decrease of noise.
The main characteristic of the iron nitride magnetic powder of JP-A-2000-277311 is the high saturation magnetization, and the magnetic powder produced in the Examples had a saturation magnetization of 190 to 200 Am2/kg (190 to 200 emu/g). However, the magnetic powder having such a very large saturation magnetization may not be suitably used in a magnetic recording medium for high density recording, because when the saturation magnetization is too high, the recording medium has too large saturation magnetization so that the recording demagnetization becomes remarkable. This tendency becomes more remarkable as a recording wavelength is made shorter. Therefore, magnetic powder of JP-A-2000-277311 is not suitable for high density recording.
In particular, in the case of high density magnetic recording media, it is essential to adequately decrease the saturation magnetization of a magnetic powder and decrease the thickness of a magnetic layer in order to decrease the recording demagnetization. As a magnetic flux density decreases, a magnetic flux from the surface of a magnetic recording medium deceases and in turn a reproducing output decreases. With the recent astonishing progress of magnetic head technology such as MR heads, recorded signals can be reproduced with a high sensitivity even with the small magnetic flux. Therefore, to achieve the high density recording, it is now necessary to increase a coercive force while setting the saturation magnetization of magnetic powder at a suitable value lower than a value which has been required.